This invention relates to combustors, and more particularly, to gas turbine combustors.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Aircraft are governed by both Environmental Protection Agency (EPA) and International Civil Aviation Organization (ICAO) standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) from aircraft in the vicinity of airports, where they contribute to urban photochemical smog problems. Most aircraft engines are able to meet current emission standards using combustor technologies and theories proven over the past 50 years of engine development. However, with the advent of greater environmental concern worldwide, there is no guarantee that future emissions standards will be within the capability of current combustor technologies. New designs and technology will be necessary to meet more stringent standards.
In general, these emissions fall into two classes: those formed because of high flame temperatures (NOx), and those formed because of low flame temperatures which do not allow the fuel-air reaction to proceed to completion (HC and CO). A small window exists where both pollutants are minimized. For this window to be effective, however, the reactants must be well mixed, so that burning will occur evenly across the mixture without hot spots, where NOx is produced, or cold spots, where CO and HC are produced. Hot spots are produced where the mixture of fuel and air is near a specific ratio where all fuel and air react (i.e. no unburned fuel or air is present in the products). This mixture is called stoichiometric. Cold spots can occur if either excess air is present in the products (called lean combustion), or if excess fuel is present in the products (called rich combustion).
Modern gas turbine combustors consist of between 10 and 30 mixers, which mix high velocity air with a fine fuel spray. These mixers usually consist of a single fuel injection source located at the center of a device designed to swirl the incoming air to enhance flame stabilization and mixing. Both the fuel injector and mixer are located on the combustor dome. In general, the fuel to air ratio in the mixer is rich. Since the overall combustor fuel-air ratio of gas turbine combustors is lean, additional air is added through discrete dilution holes prior to exiting the combustor. Poor mixing and hot spots can occur both at the dome, where the injected fuel must vaporize and mix prior to burning, and in the vicinity of the dilution holes, where air is added to the rich dome mixture. Properly designed, rich dome combustors are very stable devices with wide flammability limits and can produce low HC and CO emissions, and acceptable NOx emissions. However, a fundamental limitation on rich dome combustors exists, since the rich dome mixture must pass through stoichiometric or maximum NOx producing regions prior to exiting the combustor. This is particularly important as the operating pressure ratio (OPR) of modern gas turbines increases for improved cycle efficiencies and compactness, the combustor inlet temperatures and pressures increase the rate of NOx production dramatically. As emission standards become more stringent and OPR""s increase, it appears unlikely that traditional rich dome combustors will be able to meet the challenge.
Lean dome combustors have the potential to solve some of these problems. One such current state-of-the-art design of lean dome combustor is referred to as a dual annular combustor (DAC) because it includes two radially stacked mixers on each fuel nozzle which appears as two annular rings when viewed from the front of the combustor. The additional row of mixers allows the design to be tuned for operation at different conditions. At idle, the outer mixer is fueled, which is designed to operate efficiently at idle conditions. At higher powers, both mixers are fueled with the majority of fuel and air supplied to the inner annulus, which is designed to operate most efficiently and with few emissions at higher powers. Such a design is a compromise between low NOx and CO/HC. While the mixers have been tuned to allow optimal operation with each dome, the boundary between the domes quenches the CO reaction over a large region, which makes the CO of these designs higher than similar rich dome single annular combustors (SAC""s). This application, however, is quite successful, has been in service for several years, and is an excellent compromise between low power emissions and high power NOx.
Other recent designs alleviate the problems discussed above with the use of a novel lean dome combustor concept. Instead of separating the pilot and main stages in separate domes and creating a significant CO quench zone at the interface, the mixer incorporates concentric, but distinct pilot and main air streams within the device. However, the simultaneous control of low power CO/HC and smoke emission is difficult with such designs because increasing the fuel/air mixing often results in high CO/HC emissions and vice-versa. The swirling main air naturally tends to entrain the pilot flame and quench it. To prevent the fuel spray from getting entrained into the main air, the pilot establishes a narrow angle spray. This results in a long jet flames characteristic of a low swirl number flow. Such pilot flames produce high smoke, carbon monoxide, and hydrocarbon emissions and have poor stability.
In an exemplary embodiment, a combustor operates with high combustion efficiency and low carbon monoxide, hydrocarbon, and smoke emissions. The combustor includes a fuel injector for injecting fuel into the combustor, a baseline air blast pilot splitter including a downstream side which converges towards a center body axis of symmetry, and a splitter extension. The splitter extension includes a diverging upstream portion attached to the pilot splitter, a diverging downstream portion, and an intermediate portion extending between the upstream portion and the downstream portion.
The splitter extension increases an effective pilot flow swirl number for an inner and an outer vane angle. The increased effective swirl number results in a stronger on-axis recirculation zone. Recirculating gas provides oxygen for completing combustion in the fuel-rich pilot cup, creates intense mixing and high combustion rates, and burns off soot produced in the flame. The splitter extension enables a swirl stabilized flame with lower vane angles. The splitter extension also decreases the velocity of pilot fuel being injected into the combustor and the velocity of the pilot inner airflow stream. The lower velocities improve fuel and air mixing, and increase the fuel residence time in the flame. Fuel entrainment and carryover in the pilot outer airflow stream are also decreased by the splitter extension. Lastly, the splitter extension physically delays the mixing of the pilot inner and outer airflows causing such a mixing to be less intense due to the lower velocities of the pilot airflows at the exit of the splitter extension. As a result, a combustor is provided which operates with a high combustion efficiency while maintaining low carbon monoxide, hydrocarbon, and smoke emissions.